This invention relates to an entrance bushing, and more specifically relates to the combination of an air entrance bushing and a gas-insulated bus wherein the largest inside diameter of the porcelain portion of the bushing approximately equals the diameter of the gas-insulated bus outer housing.
Gas-insulated transmission systems are well known wherein a high-voltage substation for dealing with voltages which may be of the order of 500,000 volts and higher have their principal components, such as circuit breakers, switches, and the like enclosed in gas-filled grounded metal enclosures which are connected to one another by elongated conductors which are also contained in an outer grounded enclosure filled with a compressed insulating gas such as sulfur hexafluoride.
It is necessary to provide an air entrance bushing to make the connection from an overhead line, for example, to the central gas-insulated conductor of the gas-insulated bus. At the present time, these bushings consist of an elongated porcelain body of tapering or straight diameter with suitable corona rings at the ends of the porcelain body to allow adequate flaring of the dielectric potential lines.
The large diameter end of the porcelain body is made larger than the outer diameter of the gas-insulated bus outer housing so that the gas-insulated bus outer grounded housing can penetrate into the interior of the bushing porcelain, with a corona ring provided at the end of the grounded outer housing to provide proper termination of the grounded bus within the porcelain body. The interior volume of the outer housing bus then communicates with the interior volume of the porcelain of the bushing so that the porcelain can be filled with gas under pressure to ensure good dielectric isolation between the bushing central conductor and its outer insulation components.
The cost of the porcelain portion of the air entrance bushing is generally proportional to the square of its largest diameter. Since prior art bushings require the outer housing of the gas-insulated bushing to enter the bushing, the bushing outer diameter is necessarily larger than the housing diameter, thus substantially setting a high minimum cost for the air entrance bushing. Moreover, the larger diameter needed for the bushing requires larger wall thicknesses for the porcelain in order to withstand pressure loading on the bushing from the high-pressure dielectric gas. This increased wall thickness further increases the cost of the bushing.
The presently used construction for the air entrance bushing of a gas-insulated bus also permits the inadvertent deposition of metal particles and other contamination products on the surfaces of the spacer insulators, thereby to reduce the electrical withstand level of the bus. It is extremely difficult to reduce the contamination level within the insulator bushing by preassembly of the bushing at the factory since transportation considerations require the assembly of the device in the field. Thus, contamination problems within the air entrance bushing of a gas-insulated system are increased.